Many ladar and optical sensors have been developed for use by spacecraft for rendezvous and docking missions. Some of these are discussed briefly below.
Sandia National Labs has, for instance, developed a scannerless rangefinder that can produce high density depth maps at high rates. The range imager apparently works by using a high-power laser diode to illuminate a target. The phase shift of the reflected light from the target relative to the AM carrier phase of the transmitted light is apparently measured to compute the range to the target. The gain of the image intensifier within the receiver is modulated at the same frequency as the transmitter. The light reaching the detector is typically dependent on the phase of the return signal and its intensity may also be dependent upon the reflectivity of the target. To normalize reflectivity variations the intensity of the return beam may be sampled twice, one with the receiver modulation gain disabled and once with the modulation on. Thus, the range associated with each pixel is essentially measured simultaneously across the entire scene. This is a relatively short range sensor (46-300 m) and is typically only suitable for inspection purposes. A complete system would typically require the addition of another sensor such as a radar system.
The LDRI is described in, for instance, U.S. Pat. No. 6,677,941 issued to Lin on Jan. 13, 2004 entitled “Three-dimensional relative positioning and tracking using LDRI”, the contents of which are hereby incorporated by reference.
The Rendezvous Radar (RVR) for Engineering Test Satellite seven (ETS-VII), launched by the National Space Development Agency of Japan (NASDA) on Nov. 28, 1997 to conduct the space robot technology experiments, is, apparently, an optical navigation sensor which will be used for distances from about 2 m to about 600 m. The RVR emits a 810 nm laser pulse and measures the reflected light from a cubed corner reflector. This is an extremely short range sensor and requires the target to be equipped with cubed corner reflectors, a major disadvantage.
Optech and MD Robotics have developed a Rendezvous Laser Vision System (RELAVIS) to address on-orbit servicing requirements. RELAVIS is similar to the commercially produced ILRIS-3D. Preliminary tests apparently demonstrate a maximum range of about 2.5 km with range accuracy of about 1 cm for the entire range and positional accuracy of about 2 cm. This sensor typically does not require retroreflectors but because of its short range typically must be supplemented by other typically expensive sensors such as radar.
Orbital Sciences has built the Advanced Video Guidance Sensor for use on the NASA DART mission. The sensor is based on the Video Guidance Sensor (VGS) and Advanced Video Guidance Sensor (AVGS) developed by NASA/MSFC for use in space rendezvous and docking. The AVGS fires lasers of two wavelengths, 800 nm and 850 nm at retroreflective targets on the chase vehicle. The retro-reflective targets are shielded with an optical filter that allows only the 850 nm wavelength laser to be reflected. Thus subtraction of the 800 nm image from the 850 nm image highlights the illuminated targets in all lighting conditions. AVGS software generates centroids for each of the targets. The geometric arrangement of the targets allows determination of relative position and orientation. The targets do not have to be in any specific pattern, aside from not being coplanar. At long range a set of widely-space targets are used. At shorter range a cluster of targets are used. This permits the use of the sensor at ranges of 100 s of meters yet preserves precision at closer ranges. The accuracy ranges from 10 mm along the perpendicular and 0.75 deg at 5 m range to 3 mm along the perpendicular and 0.3 deg at less than 3 m. The AVGS/VGS system uses predefined points on the target. In addition, it employs controlled illumination to improve the detection of the points. This simplifies the video processing considerably at the expense of adding the lasers for illumination and retro-reflectors on the target vehicle. This device has limited range and requires retroreflectors on the target spacecraft. Thus, it requires additional sensing means for long range detection.
U.S. Pat. No. 6,411,871 by Ching-Fang Line dated Jun. 25, 2002, the contents of which are hereby incorporated by reference, appears to describe an autonomous navigation, guidance, and control process for docking and formation flying by utilizing the laser dynamic range imager (LDRI) and other key technologies, including fuzzy logic based guidance and control, optical flow calculation (including cross-correlation, phase-correlation, and image differential), software design and implementation, and verification methods. The autonomous navigation, guidance, and control process includes the steps of providing an expected reference position relative to a target; generating a carrier position and attitude relative to said target by a Range and Intensity Images Provider; producing a relative position and attitude error; and producing control commands from said relative position and attitude error for relative motion dynamics. This sensor can only be used at short range and must be augmented by other long range sensors.